In linguistics, the lexicon (or wordstock) of a language is its vocabulary, including its words and expressions. A lexicon is also a synonym of the word thesaurus. More formally, it is a language's inventory of lexemes. Coined in English 1603, the word "lexicon" derives from the Greek "λεξικόν" (lexicon), neut. of "λεξικός" (lexikos), "of or for words",[1] from "λέξις" (lexis), "speech", "word",[2] and that from "λέγω" (lego), "to say", "to speak".[3]

The lexicon includes the lexemes used to actualize words. Lexemes are formed according to morpho-syntactic rules and express sememes. In this sense, a lexicon organizes the mental vocabulary in a speaker's mind: First, it organizes the vocabulary of a language according to certain principles (for instance, all verbs of motion may be linked in a lexical network) and second, it contains a generative device producing (new) simple and complex words according to certain lexical rules. For example, the suffix '-able' can be added to transitive verbs only, so that we get 'read-able' but not 'cry-able'.

Usually a lexicon is a container for words belonging to the same language. Some exceptions may be encountered for languages that are variants, like for instance Brazilian Portuguese compared to European Portuguese, where a lot of words are common and where the differences may be marked word by word.

When linguists study the lexicon, they study such things as what words are, how the vocabulary in a language is structured, how people use and store words, how they learn words, the history and evolution of words (i.e. etymology), types of relationships between words as well as how words were created.

An individual's mental lexicon, lexical knowledge, or lexical concept is that person's knowledge of vocabulary. The role the mental lexicon plays in speech perception and production, as well as questions of how words from the lexicon are accessed, is a major topic in the fields of psycholinguistics and neurolinguistics, where models such as the cohort model have been proposed to explain how words in the lexicon are retrieved.